The invention relates to compositions, methods, and apparatuses for improving dry strength in paper using a process of treating pulp slurry with a combination of strength agents.
As described for example in U.S. Pat. Nos. 8,465,623, 7,125,469, 7,615,135 and 7,641,776 and U.S. patent application Ser. No. 13/962,556, a number of materials function as effective wet-end dry strength agents. These agents can be added to the slurry to increase the tensile strength properties of the resulting sheet. As with retention aids however they must both allow for the free drainage of water from the slurry and also must not interfere with or otherwise degrade the effectiveness of other additives present in the resulting paper product.
Maintaining high levels of dry strength is a critical parameter for many papermakers. Obtaining high levels of dry strength may allow a papermaker to make high performance grades of paper where greater dry strength is required, use less or lower grade pulp furnish to achieve a given strength objective, increase productivity by reducing breaks on the machine, or refine less and thereby reduce energy costs. The productivity of a paper machine is frequently determined by the rate of water drainage from a slurry of paper fiber on a forming wire. Thus, chemistry that gives high levels of dry strength while increasing drainage on the machine is highly desirable.
As described for example in U.S. Pat. Nos. 7,740,743, 3,555,932, 8,454,798, and US Published Patent Applications 2012/0186764, 2012/0073773, 2008/0196851, 2004/0060677, and 2011/0155339, a number of compositions such as glyoxalated acrylamide-containing polymers are known to give excellent dry strength when added to a pulp slurry. U.S. Pat. No. 5,938,937 teaches that an aqueous dispersion of a cationic amide-containing polymer can be made wherein the dispersion has a high inorganic salt content. U.S. Pat. No. 7,323,510 teaches that an aqueous dispersion of a cationic amide-containing polymer can be made wherein the dispersion has a low inorganic salt content. European Patent No. 1,579,071 B1 teaches that adding both a vinylamine-containing polymer and a glyoxalated polyacrylamide polymer gives a marked dry strength increase to a paper product, while increasing the drainage performance of the paper machine. This method also significantly enhances the permanent wet strength of a paper product produced thereby. Many cationic additives, but especially vinylamine-containing polymers, are known to negatively affect the performance of optical brightening agents (OBA). This may prevent the application of this method into grades of paper containing OBA. U.S. Pat. No. 6,939,443, teaches that the use of combinations of polyamide-epichlorohydrin (PAE) resins with anionic polyacrylamide additives with specific charge densities and molecular weights can enhance the dry strength of a paper product. However, these combinations require the use of more than optimal amounts of additives and are sometimes practiced under difficult or cumbersome circumstances. As a result there is clear utility in novel methods for increasing the dry strength of paper.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.